1. Technical Field of the Invention
The present invention relates to a translucent polycrystalline ceramic and a method of making the same and in particular relates to a translucent polycrystalline Al2O3 ceramic and a method of making the same.
There are many applications of the translucent polycrystalline ceramic. For instance, an outer tube for a metal halide or sodium lamp, a light emitting tube and a window plate for high-temperature furnace are made of translucent alumina ceramic, wherein not only a electrical resistance and/or chemical corrosion resistance, but also a high strength and hardness from low temperature (ambient) up to as high as 1000-1200xc2x0 C. are often required. This is partly because the translucent alumina ceramic may be thinned to gain more light transmittance therethrough. In addition to a high straight-line light transmission or rather called as liner-light transmittance (corresponding to transparency), some applications need a fine texture and high abrasion resistance (namely ceramic particles does not fall off from its surface maintaining a smooth and less roughness surface). Such applications includes optical connectors, optical filters, medical articular heads in medical use, cutting tools, bearings, high-Q dielectrics for some electronic components.
2. Related Arts
It is known that some alumina ceramics (polycrystalline) can be made translucent or partially transparent, in other words, light-transmittable therethrough.
However, the translucent alumina ceramic that satisfies both transparency and high mechanical strength has not been reported. For example, in Japanese Patent Laid-Open No. H03-285865/1991, a translucent alumina ceramic made by using alumina grains of high-purity (99.99% purity) and a small amount of MgO is disclosed, however, its bending strength is insufficiently low as about 500 MPa. On the other hand, in other Japanese Patent Nos. 2729204 and 2663191, the translucent alumina ceramic having high strength and hardness made by controlling a mean particle size under HIP is disclosed, however, a straight-line transmission of light propagating through a thickness of 1 mm thereof is insufficiently lower than 50%.
In addition, it has been conventionally considered that large particle sizes of alumina crystal grains constituting the alumina ceramic contributes to gaining a high translucency or transparency of the alumina ceramic because chances of light-scattering(:reflection and/or refraction occurring at grain boundaries by the light) are reasoned to be lesser than the alumina having much smaller grains inside.
In the conventional translucent alumina ceramic, alumina particles constituting the polycrystalline alumina ceramic are liable to fall off from its surface. In other words, it has been difficult to attain a precisely or rather extremely mirror-polished surface of the translucent alumina ceramic. When the conventional translucent alumina ceramic is subjected under a large contact-stress as in use for bearings or cutting tools or even in a surface polishing process thereof, such a stress-concentrated site thereof tends to break, probably due to large size crystals are formed with magnesia (MgO) binding the crystals inside the conventional translucent alumina ceramic.
An object of the invention is therefore to provide a translucent polycrystalline ceramic having a good strength and hardness, capable of transmitting a light through the ceramic.
Another object of the invention is to provide a translucent polycrystalline alumina ceramic having excellent strength and hardness or abrasion resistance, capable of light transmittance and/or straight-line light transmission therethrough and withstanding a high temperature.
Still another object of the invention is provide a method of making a translucent polycrystalline alumina ceramic. This method enables manufacture of an excellent translucent alumina ceramic having high temperature bending strength and hardness and/or abrasion resistance, wherein particles/grains constituting the ceramic are hard to fall off from its surface and the surface can be ground and polished into a smooth surface with less surface roughness and be mechanically less injured for instance under a high contact stress applied to the ceramic in use maintaining its translucency.
In a first embodiment of the invention, there is provided a translucent polycrystalline ceramic capable of transmitting a light that enters the ceramic, comprising crystal particles, wherein a mean facet length of the crystal particles is not longer than a maximum wave length of the light that transmits through the polycrystalline ceramic product; the mean facet length being defined as an average of lengths of sides forming polygons that appear in cross sections of the crystal particles constituting the ceramic.
A characteristic feature of the translucent polycrystalline ceramic according to the invention is that the mean facet length as defined above is shorter than a maximum wave length of the light that transmits through the translucent polycrystalline ceramic.
In an aspect, in the case that a visible light that has a wave length of about 380-780 nm, if all of the facet lengths (meaning facet side lengths of the alumina crystal particles) are less than 380 nm, highest translucency of the polycrystalline ceramic transmitting most of the visible light is attained with the translucent polycrystalline ceramic. Even when the mean facet length(meaning an average of the facet side lengths of the crystal particles) is less than 700 nm, the translucent polycrystalline ceramic with thickness of 1 mm can transmit more than 50% of the visible light therethrough, as will be later described in detail.
When the facet length is shorter, the better translucency and transparency is attained, and this is in contrast to the conventional technology that requires larger particles or grains (resulting in longer facet lengths) for attaining a better translucency through the ceramic.
In a preferred embodiment of the invention, the best translucency is attained when all the facet lengths are shorter than all wave lengths of the lights that transmit through the ceramic. The better light translucency as high as 70% is attained with the mean facet length of less than 500 nm and the best one (more than 75%) is attained with that of less than 400 nm.
In another preferred embodiment according to the invention is that the polycrystalline ceramic should is substantially poreless. In other words, a relative density of the fired ceramic should be at least 99.8% or substantially 100% with a minimum binder connecting the transparent crystal particles (or grains) formed inside the translucent ceramic. This is because the pores decrease translucency and/or transparency of the polycrystalline ceramic, and in addition decreases strength and hardness thereof. In the case of a translucency polycrystalline alumina ceramic, the density thereof should be at least 3.98 g/cm3 (substantially 100% in relative density).
A material candidate for the crystal particles is Al2O3, AlN, ZrO2, spinel and so on, so long as large optical anisotropy or crystal anisotropy is not formed with the crystal particles. In other words, if a mean aspect ratio of the crystal particles is 1-1.5 (preferably 1-1.3) and a mean particle size of the crystal particles formed inside the ceramic is not larger than about 1 xcexcm, the mean facet length becomes less than the maximum wave length or most of wave lengths of the visible light. Among them, Al2O3 (alumina or sappier) is best selected for the crystal particles. Because a crystal structure of the alumina belongs to a hexagonal system, difference of refractive index for the light between its crystal facets formed along a-axis and c-axis in crystallography is theoretically only about 0.008, which renders the reason why majority of the visible light can transmit through the translucent polycrystalline ceramic comprising crystal particles having the mean aspect ratio of 1-1.5 and the mean crystal particle size of not larger than 1 xcexcm.
In an aspect of the invention, the translucency of Al2O3 ceramic (namely, alumina including sapphire) as well as strength and hardness of the ceramic at high temperature are maintained as will be described later in detail, if a metal oxide is selected from oxides of metals belonging to IIIA and IVA groups of Periodic Table (IUPAC alt) excepting Ti and is added as a binder for binding or rather a sintering aid for sintering the crystal particles inside the ceramic. Ti is substantially excluded since a colorless or non-pigmented translucency is not obtained with the alumina ceramic containing Ti at its boundary. Other pigmenting element such as Cr and Co (although not belonging to IIIA and IVA groups of the Periodic Table) is substantially also avoided for the binder or sintering aid.
Most preferred theoretically is that the translucent polycrystalline alumina is a sintered product that is made without such a sintering aid or binder. In actual practice, the ratio of alumina occupying the translucent alumina ceramic (meaning a relative content of alumina in the sintered ceramic product) is made preferably to at least 99% or more preferably at least 99.95% in volume. To attain this, a starting material powder of alumina is selected preferably from those having the purity of not less than 99.99% or not less than 99.995% (best).
If colored translucent polycrystalline alumina ceramic is required in a application such as an optical filter, a very small amount of the pigmenting elements should be selectively added.
A soft metal oxide such as magnesia(MgO) that is conventionally used as a colorless sintering aid or binder for sintering the Al2O3 crystal particle is not recommended to be used, in the case that the strength and hardness of the translucent alumina ceramic at high temperature as high as 1000xc2x0 C. is required. This is because the MgO binder can rapidly reduces the strength and hardness at such an elevated temperature. In addition, use of the MgO binder causes the crystal particles to come off from the translucent alumina ceramic surface, rendering difficulty in attaining a smoothly polished surface of the ceramic. If in an aspect, a fine or smooth surface finish of the ceramic is not attained by polishing, the translucency of the ceramic is affected simply because of a correlation existing between a surface smoothness and translucency of the ceramic.
Since the mean particle size of the crystal particles in the translucent polycrystalline ceramic should be controlled to be small as not exceeding 1 xcexcm, a strong and hard binder for binding the crystal particles is necessary; and that is one of the reasons why the oxide of metals belonging to IIIA and/or IVA groups of the Periodic Table is used for the translucent polycrystalline alumina (including sappier) according to the invention. For instance, Y2O3, Yb2O3, ZrO2, Sc2O3, La2O3, Dy2O3 and Lu2O3 are recommended; and among them Y2O3 and/or Yb2O3 perform best.
In an aspect of the invention, an amount of the metal oxide(s) included in the transparent polycrystalline ceramic is in an amount of less than 2% in molarity (2 mol %). In order to attain the highest density of the translucent ceramic with this small amount of the metal oxide above and to attain strength and hardness, as will be described later, the ceramic is sintered under HIP (hot isostatic pressure) so that the crystal facet and the crystal particles are controlled in length and size respectively during firing (sintering) at comparatively low temperature.
Therefore, in an aspect of the invention, there is provided a translucent polycrystalline ceramic capable of transmitting a visible light that enters the ceramic, comprising Al2O3 crystal particles and a metal oxide between the crystal particles, wherein a relative density of the translucent polycrystalline ceramic is not less than 3.98 g/cm3 (or preferably 3.99 g/cm3 that is very close to its theoretical density); a bending strength of the translucent polycrystalline ceramic is more than 750 MPa; and a Vickers hardness of the translucent polycrystalline ceramic is more than 1900; and the metal oxide is an oxide of one or more metals selected from the metals belonging to IIIA and/or IV groups of the Periodic Table excluding Ti.
A better performance of the translucent polycrystalline alumina ceramic is attained if the metal oxide contained therein is 0.02-2.0% in molarity and a mean particle size of the crystal particles is 0.3-1.0 xcexcm, according to an aspect of the invention. Namely a bending strength and a Vickers Hardness thereof becomes at least 500 MPa and at least 850, respectively, measured at a temperature of 1000xc2x0 C.
This translucent polycrystalline ceramic according to the invention is capable of more than 50% of the light having a wave length of from 380-780 nm can transmit through the translucent polycrystalline ceramic when a thickness of the ceramic is 1 mm.
Further, the translucent polycrystalline ceramic has a feature of a straight-line light transmission ratio of at least 0.3 (or 30% in percentage) , which straight-line light transmission ratio is determined by dividing the light intensity transmitted through the ceramic in less than 0.5 degree angle by a total light intensity originally entering the ceramic when the ceramic is 0.5 mm in thickness and the light has a wave length of 380-780 mm.
A surface of the above translucent polycrystalline alumina ceramic according to the invention is so hard that the surface can be polished to an extent that a center line mean surface roughness (Ra) is from 0.002 to 0.020 xcexcm and a maximum height (Rmax) of the surface roughness is less than 0.30 xcexcm and/or to the extent that an empty surface area caused by the alumina particles fallen off from the surface to the total polished surface area is not higher than 1%.
Since the translucent polycrystalline alumina according to the invention has a high corrosion-resistance, it can be used for e.g. an outer tube of a sodium lamp wherein a high sodium vapor pressure is confined. This translucent polycrystalline alumina ceramic has a very high strength and hardness, it can be used for a cutting tool having an edge formed by a rake face and a flank face, or for a abrasion resistance field including bearings. Since an electrical performance is expected similar to sappier, this polycrystalline ceramic may be used as a dielectric material in various electronic components, especially in a high frequency field.
In an embodiment according to another aspect of the invention, a preferable mean particle size of alumina crystal particles constituting the translucent alumina ceramic is from 0.3 to 0.7 xcexcm. When the mean particle size of the alumina crystal particles exceeds 1.0 xcexcm, the strength and hardness of the sintered alumina particle reduces rapidly, possibly resulting in a comparatively low abrasion resistance ceramic and causing crystal particles to fall off during a surface polishing, which will injure the surface or weaken the ceramic under e.g. a continuous contact stress.
There is expected no serious problems in the properties such as strength, harness, abrasion resistance and light transmittance in the translucent polycrystalline ceramic, since it may be theoretically better in view of translucency and transparency the crystal particles and the facet lengths are controlled to be smaller than 0.3 um in size and less than 200 nm in length respectively. However, there may be a drawback that such fine particles are hard to be processed resulting in high cost for the sintered ceramic.
For cutting tools, at least 750 MPa (or more than 830 MPa or in some cases more than 1100 MPa) in bending strength and at least 1900 (more preferably more than 2100) in a Vickers hardness are normally required. The translucent polycrystalline ceramic according to the invention satisfy such requirements for cutting tools. In addition, since the alumina crystal particles are make as having a small aspect ratio of from 1.0 to 1.5, the sintered product containing the crystal particles is excellent in the abrasion resistance as well as the high strength and the high hardness. If the aspect ratio is controlled to 1.0 to 1.35, not only the abrasion resistance but also straight-line light transmittance relating to transparency of the ceramic are highly maintained.